1. Field of the Invention
The present invention relates to a process for the conversion of ethanol and water to a product rich in acetic acid. More particularly, the invention is a process for combined oxidative and non-oxidative dehydrogenation of ethanol to acetic acid.
2. Description of the Related Art
Different methods for the preparation of acetic acid from ethanol have been known for many years.
Ethanol can be produced from ethylene by hydrolysis, and it may be produced by fermentation of sugars. Typically, hydrolysis of ethylene to ethanol has been preferred primarily to meet the technical use of ethanol, while fermentation of sugar containing matter is an ancient process the product of which is primarily used for household purposes. In the latter process the ethanol produced is obtained in an aqueous solution in a concentration of 5-15% by weight along with fermentation by products and solids, the so-called broth.
Typically, the ethanol is then distilled in two columns to obtain 96% ethanol and may finally be dried in a bed of zeolites to obtain anhydrous ethanol useful as an additive to gasoline.
As part of a new fuel supply development the capacity of bio-ethanol production for the use as gasoline additive has increased tremendously over the past 10 years, especially in Brazil and the US.
Ethanol can be converted by dehydrogenation to acetic acid via the oxidative and the non-oxidative route, viz.:EtOH+O2═HOAc+H2O (oxidative route)  (1)EtOH+H2O═HOAc+2H2(non-oxidative route)  (2)
The oxidative route is exothermic (−ΔH|=439 kJ/mol) and not limited by equilibrium and the non-oxidative route is endothermic (−ΔH|=−44 kJ/mol) and equilibrium limited producing acetaldehyde as an intermediate.
It is known that e.g. copper is an active catalyst for the non-oxidative dehydrogenation of ethanol to acetic acid. Other catalysts like coal are capable to convert ethanol in the non-oxidative route to acetic acid.
Some of the catalysts active in the non-oxidative route are active also in the esterification of ethanol and acetic acid, whereby ethyl acetate constitutes part of the product composition. Typical by-products in the non-oxidative route are coupling reaction products ketone, aldehyde and alcohol products, e.g. propanon, butanal and butanol.
Examples of catalysts active in the oxidative conversion of ethanol to acetic acid are vanadium oxide, gold nanoparticles and supported palladium.
Suggestions of processes to make acetic acid from ethanol are sparse.
GB 287064 discloses an acetic acid process, where an alcohol such as ethyl alcohol is passed upward in a reactor column containing in a first bed an Ag doped Cu catalyst in its reduced state and in its oxidised state at the top of the reactor. The reduced catalyst provides for dehydrogenation of ethanol to acetaldehyde, which is oxidized by contact with Cu oxide to acetic acid being withdrawn from the top of the reactor. Cu oxide is thereby reduced to copper. The Cu catalyst recovered from the bottom of the reactor may be reoxidized and recycled to the top of the reactor. This process employs a moving bed with Cu/CuO as catalyst and an oxygen carrier for the oxidation of ethanol to acetic acid via acetaldehyde.
Kanichiro Inui et al (Effective formation of ethyl acetate from ethanol over Cu—Zn—Zr—Al—O catalyst’, Journal of Molecular Catalysis A: Chemical 216 (2004), page 147-156) describes Cu—Zn—Zr—Al—O as catalysts being active in the conversion of ethanol to ethyl acetate and to acetic acid in presence of water by non-oxidative route. It is mentioned that the selectivity to propanone decreases with increasing selectivity to acetic acid. Up to 15 wt % water in the feed is described, which corresponds to 31% on a mole basis. It is proposed that the reaction proceeds via acetaldehyde, hemiacetal and ethyl acetate to acetic acid through a final hydration.
JP 57102835 discloses a non-oxidative process for the production of acetic acid from ethanol in a first ethanol dehydrogenation reaction over a CuO and other oxidic catalysts and a hydrogen separation step. In a subsequent step acetic acid together with water is separated and acetaldehyde is separated from unconverted ethanol. This process may further comprise a second acetaldehyde dehydrogenation step to acetic acid with addition of additional water, wherein the product of this step is recycled to the hydrogen separation step and unconverted ethanol is recycled to the first ethanol dehydrogenation step.
When carrying out a non-oxidative synthesis of acetic acid from ethanol, the catalyst may be arranged in an adiabatic reactor or in a heated reactor. The adiabatic reactor type is cheap to operate, however, a large temperature decrease over the reactor results in a lower product yield or requires high internal cooling rate or recycle rate in order to limit the temperature decrease over the reactor.
The heated reactor is an expensive alternative due to its more complicated construction. Furthermore, a heat source is required to supply the heat for the reaction.
The conversion of ethanol to acetic acid via the oxidative route is strongly exothermic. Due to the strong exothermic nature the reaction must be performed in a reactor type provided with efficient heat removal means, i.e. high areas of heat transfer are mandatory, which results in an expensive design. Furthermore, risks of temperature runaway and general selectivity problems are negative results of strong exothermic reactions.
In contrast to the above discussed reaction types, a process without a heat requirement, a so called thermoneutral process, does not require special means of heat supply or heat removal in the reactor. Ideally, if a chemical process takes place at a temperature higher than ambient, a slightly exothermic process supplies heat for the preheating of the feed by heat exchange with the hot reactor effluent having a temperature higher than the feed. Processes where the adiabatic temperature increase is moderate or low are considered to be thermoneutral. In context with the invention, reactions having a ΔH| and specific heat capacity of the reactants resulting in an adiabatic change of the temperature between about −25 to 25° C. are considered thermoneutral.